In transmission systems like radio networks, e. g. digital cellular radio networks according to the GSM-standard (Global System for Mobile communications), not only user data, e. g. encoded speech signals, are transmitted but also various information necessary for the operation of the network. The transmission of these information often is referred to as signaling. Signaling messages allow the fixed part of the network to discuss management of several issues either related to the user, e. g. call in progress indications, or concerning technical aspects of the communication, e. g. preparation and execution of a handover, with the mobile part of the network. The establishment and the release of a call also require signaling exchanges. In addition, signaling exchanges are even needed in cellular radio networks between fixed components of the network and mobile stations when no communication or call is in progress.
In order to transmit signaling information in parallel with the transmission of a user data flow, GSM offers two possibilities. Each traffic channel (TCH) for transporting the user data has an associated low rate channel, used for the transport of signaling called slow associated control channel (SACCH). It is used for non-urgent procedures, mainly the transmission of the radio measurement data needed for the decisions concerning handover. Other needs of associated signaling, e. g. authentication or the command to handover, make use of the TCH itself, called fast associated signaling. For fast associated signaling a so called stealing flag is used, which indicates that a part of the information or data contained in the indicated TCH frame contains signaling information. The stealing flag indicates that either the first half or the complete TCH frame contains signaling information.
The known methods for signaling information in a radio network do have certain disadvantages, like the use of an additional channel, e.g. the SACCH channel. It is another disadvantage, that if the traffic channel itself is used, at least half of the bits of each TCH frame are used for signaling and therefore are no longer available for the transmission of user data. It is a further disadvantage that signaling information being transmitted within one frame is susceptible to errors being caused by bad transmission conditions.